When a single creature, or a whole species, changes in order to better fit their environment it is called

    The right answer to this question is: Adaptation

Explanation:

    Adaptation is based on Darwin's theory, and it's not that simple. The adaptation of a species to fit an environment is almost always through random mutation, and these mutations can or cannot help the organism, it is completely random, but when it is good for them, this mutation stays on this population until it's not needed anymore.

      Darwin's theory is called Natural Selection, but nowadays, this theory has changed a lot with time, and genetics can now contribute to the explanation.

your answer is going to be physical weathering

fumikage isn't really a bird, and tsuyu does have teeth. tsuyu has a transformation quirk which still allows her to be partially human. otherwise we'd have a large frog running around. fumikage has teeth because again, he isn't a bird. his head was a quirk mixture if i'm correct.